In the conventional vehicles, a transmission for changing speed of the engine serving as a prime mover and an engagement device in which a torque transmitting capacity thereof is varied in accordance with an engagement pressure are controlled hydraulically. To this end, the vehicle of this kind is provided with a mechanical oil pump that is connected to a rotary member rotated together with an engine to be driven. In order to control the transmission and the engagement device hydraulically, a line pressure established by the oil discharged from the mechanical oil pump is regulated to a predetermined initial pressure. The surplus oil discharged to regulate the line pressure may be delivered to a torque converter or a lubrication site.
In the conventional art, a so-called “economy running system” or a “stop-start system” have been in practice to save fuel by stopping the engine when the engine power is required neither to rotate drive wheels nor to activate auxiliaries such as an alternator for generating electric power.
Given that the engine is stopped by such an automatic stopping system in the vehicle having the mechanical oil pump driven by the engine, pressurized oil will not be discharged from the mechanical oil pump during stopping the engine. In this situation, a speed change operation of the transmission and an engagement of the engagement device may not be carried out. In order to avoid such disadvantages, the conventional vehicle of this kind is further provided with an electric motor that is activated independently from the engine, and an electric oil pump that is driven by the output torque of the electric motor.
An example of the hydraulic control system for vehicles having the mechanical oil pump and the electric oil pump is disclosed in Japanese Patent Laid-Open No. 2010-209991. The control device taught by Japanese Patent Laid-Open No. 2010-209991 is configured to deliver oil to a hydraulic actuator for changing a speed ratio from the electric oil pump, and to prevent the oil discharged by the electric oil pump from flowing toward to a cooling site and a lubrication site during stopping the engine automatically. Specifically, the control device is configured to deliver the oil from the mechanical oil pump to the hydraulic actuator through a check valve and a control valve, and to deliver the oil from the electric oil pump to a passage connected to the control valve and the actuator through the check valve. Thus, in the hydraulic control device of this kind, the pressurized oil is delivered to the hydraulic actuator from the electric oil pump when the mechanical oil pump is inactivated during stopping the engine automatically. That is, the hydraulic pressure applied to the hydraulic actuator to carry out a speed change operation can be controlled even during stopping the engine automatically, by controlling the pressure of the oil discharged from the electric oil pump, or by draining the oil when the pressure in the hydraulic actuator is increased by controlling the control valve. In addition, since the electric oil pump is not required to deliver the oil to the cooling site and the lubrication site, the electric oil pump may be downsized.
However, according to the hydraulic control device taught by Japanese Patent Laid-Open No. 2010-209991, air may remain in the oil passage in an upstream side of the electric oil pump when the electric oil pump is started. Given that the electric oil is driven to apply the hydraulic pressure to the hydraulic actuator in this situation, controllability of the hydraulic actuator may be degraded. In addition, given that the control valve is opened to discharge air from the passage in the upstream side of the electric oil pump when starting the electric oil pump, the hydraulic pressure of the hydraulic actuator cannot be increased rapidly during discharging the air. That is, hydraulic response of the hydraulic actuator would be slowed.